In the manufacture of optical fiber cable, a glass preform rod which generally is manufactured in a separate process is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and optical fiber is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower. Subsequently, one or more optical fiber is provided with a sheath system to form an optical fiber cable.
Because the surface of the optical fiber is very susceptible to damage caused by abrasion, it becomes necessary to coat the optical fiber, after it is drawn, but before it comes into contact with any surface. Inasmuch as the application of the coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must become solidified rapidly before the optical fiber reaches a capstan. This may be accomplished by photocuring, for example. The refractive index of the cured coating should desirably be above that of the outer layer of glass of the optical fiber.
Optical fiber performance properties which are affected most by the coating material include strength, resistance to microbending transmission loss, strippability and abrasion resistance. Coating defects which may expose the optical fiber to subsequent damage arise primarily from improper application of the coating material. Defects such as large bubbles or voids, non-concentric coatings with unacceptably thin regions, or intermittent coatings must be avoided. When it is realized that the coating thickness may be as much as the radius of an optical fiber, it becomes apparent that non-concentricity can cause losses in splicing, for example.
Transmission losses, for example, may occur in optical fibers because of a mechanism known as microbending. Optical fibers are readily bent when subjected to mechanical stresses, such as those encountered during placement in a cable or when the cabled fiber is exposed to varying temperature environments or mechanical handling. If the stresses placed on the fiber result in bending distortion of the fiber axis with periodic components typically ranging from the micron to the centimeter range, light propagating in the fiber core may escape therefrom. These losses, termed microbending losses, may be very large. Accordingly, the fiber must be isolated from stresses which cause microbending. The properties of the fiber coating play a major role in providing this isolation.
Two types of coating systems have been used to overcome this problem. Single coatings, employing a relatively high shear modulus, e.g. over a range of about 1000 to 500,000 psi, have been used in applications requiring high fiber strengths or in cables which employ buffer tubes where fiber sensitivity to microbending is not a significant problem.
Dual coated optical fibers typically are used in cables to obtain design flexibility and improved performance. Typically, a dual coated optical fiber which includes a coating system comprising an inner or primary coating layer characterized by a relatively low modulus material is applied to the optical fiber. The modulus of the primary coating should be effective in reducing the stress transmitted to the glass by an external lateral force thus reducing microbending of the glass. Primary coating materials have been characterized by an equilibrium modulus of elasticity in the range of about 50 psi to 200 psi. Equilibrium modulus may be defined as the final modulus that a crosslinked material will reach in time or at high temperatures. This modulus is chosen so that the primary coating achieves its principal purpose, i.e., the attenuation and uniform distribution of stress supplied to the fiber. Through this attenuation and distribution, losses due to microbending are substantially reduced. See L. L. Blyler, Jr. and C. J. Aloisio, ACS Symposium, Series No. 285, Applied Polymer Science, pp. 907-930, 1985, for a description and definition of microbending losses. Clearly, the primary coating itself should not introduce excessive stress during application or use. Such a material reduces microbending losses associated with the cabling, installation or environmental changes during the service life of the optical fiber. In order to meet temperature conditions in expected areas of use, the low modulus coating material desirably should be effective in the range of about -50.degree. to 85.degree. C.
Typically, the primary coating material also should yield, upon exposure to actinic radiation, a layer that is adherent to the optical fiber, i.e., requires at least 500 and preferably 1000 grams force for separation from the fiber. However, that value should be less than 3200 grams to facilitate removal and to avoid tenacious residues.
An outer or secondary coating layer typically comprising a relatively high modulus material is applied over the primary layer. The outer coating layer is usually of a higher modulus material to provide abrasion resistance and low friction for the coated fiber. The dual coating materials serve to cushion the optical fiber by way of the primary layer and to distribute the imposed forces by way of the secondary layer, so as to isolate the optical fiber from bending moments.
The properties of adhesion and appropriate modulus behavior are necessary to yield a suitable fiber. However, for ease of fabrication, it is also desirable that the primary coating material cure to an appropriate equilibrium modulus level over a wide range of relatively low doses. Dose is defined as the incident radiation or quantity of radiant energy impinging on the coating system. The physical volume in which an exposure source, e.g., a lamp emitting ultraviolet light (UV), for example, can be placed on a fiber drawing and coating apparatus is limited. This limitation, in turn, restricts the dose available for curing and necessitates the use of a material that cures at low doses. Additionally, since the intensity of radiation sources employed for curing are generally not variable, any change in drawing speed produces a concomitant change in dose. Aging of the radiation source and/or deposition of materials on its external surfaces also induces dose variation. Accordingly, to produce a consistently acceptable fiber, the primary coating should cure over a wide range of low doses to within a specified range of equilibrium modulus. A coating whose modulus strongly varies with curing dose will result in a wide variation in modulus range and fiber performance.
The coating materials should be characterized by predetermined properties in order to achieve desired performance characteristics. Properties which provide strength must characterize the coated optical fiber while not adding to the transmission loss performance of the optical fiber. Further, the coating system must be such that it is strippable from the underlying glass without undue force or tenacious residues and such that the stripped fiber is capable of easily being terminated by any of a variety of arrangements available in the marketplace. Also, the primary coating material should have a suitable refractive index which is higher than that of the cladding. Both the primary and the secondary coating materials should be as hydrophobic as possible to prevent the accumulation of moisture at the interface between the primary coating material and the glass or cause a phase separation into water-rich regions within the material. The primary coating material should have suitable microbend resistance maintained to low temperatures to avoid transmission losses in cables. The secondary coating material should have suitable microbend resistance, abrasion and cut-through resistance and require a reasonably low strip force for removal. Of course, the cure speed of the coating system is very important in the manufacture of optical fiber to produce a consistent product within allowable manufacturing variations. The lowest UV dose at which the coating modulus is in the specified range of values is considered its cure speed.
All the above properties have been known for some time and yet fiber performance has been affected adversely by coating systems in use today. What seemingly has not been done is to determine the interrelationship of various properties of coating materials so that coating materials are characterized simultaneously by a set of properties which are optimized but not necessarily maximized.
For coating materials of the prior art, the change of properties with time is more than desired. Cables are designed according to coating properties at fiber draw. If those properties change with respect to time, the selected cable design may be inadequate and cause problems. Hence, it should be apparent that the absence of change with aging is important. In other words, the requirements must be met by specific compositions over a range of temperatures and humidity. Prior art coating systems do not appear to provide stability over time and throughout these ranges of conditions. For example, they do not age well in hot, humid conditions. Under such conditions, the properties of commercially available prior art coating materials change. In other words, under those conditions, the prior art coating materials experience chemical instability. Further, the adhesion and the glass transition temperature change over time, yellowing occurs and the coating material becomes opaque because of separation of water-rich regions due to moisture absorption.
Also, the prior art coating materials cure incompletely at relatively low doses. In an effort to reduce costs, it becomes an objective to provide coating materials which include more reactive ingredients to facilitate more rapid curing of the materials. It also is important in manufacture that the coating material undergo substantial cure so that the amount of non-reacted materials is minimized. Non-reacted materials diffuse out with age, causing a change in properties such as surface quality resulting in a tacky surface, for example.
What is needed and what seemingly is not available in the prior art is an optical fiber cable which includes coated optical fiber including properties which characterize its coating system to provide desired performance characteristics. Included in those characteristics are low loss, suitable strength, suitable strippability and adequate cut-through resistance. Needed also are a modulus spectrum and a glass transition temperature which together with elongation and adhesion, for example, provide the desired characteristics. Although some of those property demands have been achieved piecemeal to achieve specific desired performance characteristics, the art has not recognized nor provided a global solution as to how to achieve the totality of properties. What is sought is a solution set of properties which must be met by a particular composition or compositions of a coating system of an optical fiber if that system is to meet desired performance characteristics.